Only define the MSVC C++11 feature macros in C++

Don't define them if qcompilerdetection.h was compiled in C mode.

Change-Id: I080b62ef7c68bb582e55e9e3a1dff4e6c1bb48bd
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/corelib/global/qcompilerdetection.h

@@ -785,6 +785,7 @@
 #endif
 
 #if defined(Q_CC_MSVC) && !defined(Q_CC_INTEL)
+#  if defined(__cplusplus)
 #    if _MSC_VER >= 1400
        /* C++11 features supported in VC8 = VC2005: */
 #      define Q_COMPILER_VARIADIC_MACROS
@@ -843,6 +844,7 @@
 #      define Q_COMPILER_INITIALIZER_LISTS
 #    endif /* VC 12 SP 2 RC */
 
+#  endif /* __cplusplus */
 #endif /* Q_CC_MSVC */
 
 #ifdef __cplusplus